The constantly increasing bandwidth demand is motivating the telecommunication operators to expand their phase-optical networks rapidly. Accordingly, optical circuits in networks in which signals are processed completely optically are increasing in importance. The application of optical circuits makes it possible to reconfigure or restore a network in the optical sector. The chosen construction of such circuits comprises integrated planar waveguides in order to keep the space requirement needed for this purpose as small as possible so that a plurality of such circuits can be linked together relatively easily. A switching matrix can consequently be constructed on a single compact optical component.
An important element of many such switching matrices is the integrated directional coupler. In this connection, a plurality of waveguides, generally two, is structured on a substrate, for example of silica, in such a way that they have a coupling region. Said coupling region comprises waveguide segments that extend sufficiently near to one another in order to make it possible for optical signals that are conducted along an optical waveguide to be coupled over completely or partly into the adjacent optical waveguide in the coupling region.
The first directional couplers were produced in non-integrated technology with the aid of fibres. For example, DE 37 13 658 discloses a bundle of optical fibres that are first closely combined in the coupling region (for example by twisting the fibres together). They are then heated until they melt in this region and drawn apart. As a result of this drawing process, the fibres in the coupling region become thinner and the cores of the fibres move closer together. In this process, the cross-sectional areas of the individual fibres decrease. Mention is made of “taper”. However, it has emerged that the production of purely optical-fibre fused couplers is extremely difficult, particularly if it is to be completely reproducible.
This explains, inter alia, the effort made to copy optical directional couplers with the aid of integrated waveguides. JP 030071119 discloses a typical example of a directional coupler that comprises two optical waveguides integrated on a substrate. In this case, the optical waveguides extend approximately in parallel and, at the outputs, are at a sufficiently large spacing between the two optical waveguides to guarantee connection of optical components without difficulty. Typically, the directional coupler has a coupling region in which the two optical waveguides are structured sufficiently closely in order to make possible a light propagation from one waveguide to the other waveguide. The spacing of the two optical waveguides in the coupling region is for this purpose smaller by a multiple than the distance of said two optical waveguides at their outputs. Accordingly, the integrated optical waveguides are provided with a curvature in a transition region to the coupling region. Said curvature depends directly on how great the difference is between the spacing of the optical waveguides at their ends and the spacing in the coupling region. In addition, it is also determined by the area of the optical component that is available for this purpose.
Such directional couplers with integrated optical waveguides are produced, for example, in the current SiO2/Si technology. In the latter, a layer (“buffer layer”) of, for example, 15 μm made of SiO2 is grown on a silicon substrate by oxidation under high-pressure vapour. It serves to isolate the silicon substrate, which has a very high refractive index. A second layer (“core layer”) made of glass doped, for example, with phosphorous and germanium is deposited with the aid of flame hydrolysis (“flame hydrolysis deposition”—FHD) or of plasma deposition (“plasma enhanced chemical vapour deposition”—PECVD) on the oxide. In this latter layer, the optical waveguides are suitably structured, for example by dry etching. Then they are covered with a layer having a thickness of several μm of glass doped with phosphorous or boron. A typical width of the cross section of such structured optical waveguides is in the 5 to 10 μm range. Under these circumstances, the difference between the refractive index of the integrated optical waveguide and the refractive index of the surroundings is approximately 8×10−3. In order to be able to conduct light, the refractive index of the integrated optical waveguide is known to be the greatest. In the case of optical fibres, this difference is normally only half as great, i.e. 4×10−3.
An increase in the complexity of integrated optical components containing such optical directional couplers inevitably has the result that the efforts aimed at a compact construction become ever more important. The curved regions of the waveguides in the transition region to a coupling region cover the main proportion of the area of such integrated optical components. This is a direct consequence of the conditions to be fulfilled of a spacing between fibre inputs or fibre outputs of approximately 250 μm. Accordingly, the main limitation on the reduction in the area of integrated optical circuits is defined by the radius of curvature of the waveguides in the transition region to the coupling region of the directional coupler.
The smaller difference in refractive indices between core and cladding of, for example, 4×10−3 in standard optical fibres makes possible a radius of curvature of at least 10 mm. A higher curvature (smaller radius of curvature) would result in higher losses of the transmitted optical signals as a result of the optical radiation spreading in the cladding. On the other hand, a higher difference in refractive indices, as is the case for integrated optical waveguides, makes it possible to choose a smaller radius of curvature. Accordingly, the regions at the transition to the coupling region of the directional couplers comprising such integrated optical waveguides are specified as smaller than for directional couplers made of optical fibres.
On the other hand, a higher difference in refractive indices results in a stronger guidance of the optical modes. As is known, this results in higher losses at the ends of the integrated optical waveguides that have to be coupled to optical fibres. For this purpose, the ends of the integrated optical waveguides therefore have a tapered structure (“tapering”) in order to guarantee an increase in the modal field to the respective optical fibres. However, with a higher difference in refractive indices, the coupling between the two integrated optical waveguides in the coupling region is also reduced. In order not to increase the coupling region unduly as a result, the chosen spacing between the two integrated optical waveguides in the coupling region has to be smaller. This results in stricter conditions relating to the manufacturing tolerances in the production of directional couplers formed from integrated optical waveguides.